Lead frames, which form a fundamental component of semiconductor packages, are formed from conductive metal films or strips which are manufactured through photolithographic etching processes, metal punching or stamping processes, or other processes. The starting material for such processes generally involves a single strip of metal from which the lead frame is manufactured. The lead frame forms a primary interconnect component in plastic encapsulated semiconductor packages.
After the process which creates the lead frame, the lead frame may then be plated with a metallic plating which then is wire bonded to a silicon die. The silicon die, the lead frame, and the wire bonding are encapsulated together within a molded package, which may consist of an epoxy molding compound.
The purpose of the metallic plating upon the lead frame is to enhance or otherwise increase the conductivity of the lead frame. Lead frames that are poor in electrical conductivity, poor in magnetic permeability, and which have undesirable inductance characteristics will in turn have a poor signal speed therefrom. By metallic plating of the lead frames having such deleterious characteristics, a better signal speed can be achieved if the metallic plating is a better electrical conductor, has good magnetic permeability, thereby lowering the inductance of the lead frame itself.
The skin depth of the metallic plating on the lead frame is a relevant factor to the signal speed in that the current density will be greater in the plating than in the lead frame. In a principle called skin effect, the electrons in the signal have a propensity to travel on the outside of a surface of a conductor. Due to the magnetic domain of the material, the tightness of the packing of the electrons on the outside surface of the material is dependant upon the material properties of the conductor. Ultimately, the conductivity and skin depth of the plating on the lead frame will dictate the speed of the signal through lead frame.
With an improved signal speed, a semiconductor package into which the lead frame is incorporated will be able to respond to devices having a similar signal speed. For example, a microprocessor having a fast clock speed requires equally fast memory devices having fast signal speeds in order to keep up with the clock speed of the microprocessor. By maximizing the signal speed of lead frames in memory devices, a processing bottleneck is less likely to occur and speed compatibility with an associated microprocessor will be achieved.
An example of metallic plating for lead frames is silver plating. Silver is known to be a fair conductor and accomplishes a reduction of the inductance of a less conductive lead frame. A problem exists with silver plating of lead frames due to migration of the silver. This migration problem manifests itself in the formation of whiskers or thin crystal-like structures of silver that grow between two biased leads so as to cause a short.
Another problem with silver plating is that, as a precious metal, it is expensive and adds significantly to the cost in manufacturing the plated lead frame. Thus, there is a natural tendency to disfavor semiconductor packages having silver plating on or near the leads external to the package thereon which may be the source of a silver migration problem.